Pebble furnaces and method of heating pebbles



R. R. Goms 2,776,825

PEBBLE FURNACES AND METHOD 0F HEATING PEBBLES 2 Shets-Shee;

Jan. 8, 1957 Filed June 5, 1952 INVENTOR. R. R. GOINS ATTORNEYS /f/f/////U/ n 1 l Q Jan. 8, 1957 R. R. Goms 2,775,825

PEBBLE FURNACES AND METHOD 0F HEATING PEBBLES Filed June 5 1952 2SheetS-Sheet 2 FIG. 4.

F /G. 3. JNVENToR. R. R. GOINS ATTORNEYS United States Patent O ANDMETHOD OF HEATING PEBBLES Application .lune 5, 1952, Serial No. 291,88416 Claims. (ci. l26a- 19) PEBBLE FURN ACES This invention relates toimproved pebble heater apparatus and to improved methods of heatingpebbles.

This is a continuation-in-part of application Serial No. 90,921, filedMay 2, 1949, now Patent No. 2,635,990.

Pebble heat-exchangers are finding increasing favor in heating gases,such as air, nitrogen, etc., and in effecting various chemical processesin the vapor phase at elevated temperatures, especially hydrocarbonconversion processes. A conventional pebble heat-exchanger unitcomprises a pair of vertically aligned pebble heat-exchangers connectedby an axially positioned throat therebetween so as to provide forgravity ow of a contiguous stream of heat-exchange pebbles through theheat-exchangers. The pebbles in the upper heat-exchanger, or furnace,are contacted with hot combustion gas (or other hot gas) formed eitherin burners adjacent the base of the upper heat-exchanger or directly inthe pebble mass itself. ln this manner, the pebbles are heated to anysuitable temperature above the required heating or reaction temperaturein the lower heat-exchanger. The hot pebbles then gravitate to the lowerchamber where they are contacted with the feed gas to be heated,treated, and/or reacted. The lower heat-exchanger usually has afunnel-shaped bottom converging to a pebble conduit which leads to thebottom of a pebble elevator or lift, usually of the bucket type, butwhich may be of the screw or air-lift type. The elevator transfers thepebbles to a pebble chute above the level of the top of the upperheat-exchanger and this pebble chute or conduit leads into the upperportion of the upper heat-exchanger. This arrangement provides forcontinuously heating one section of a gravitating mass of pebbles andsimultaneously continuously heating a gas in the lower section of thegravitating mass of pebbles and returning the cooled pebbles from thelower section of the gravitating mass to the upper section for repeatingthe cycle.

Pebbles utilized in the process and apparatus with which this inventionis concerned are compact, spherical units consisting of alumina,mullite, zirconia, thoria, periclase, magnesia, high temperature alloys,such as Monel and Inconel, and in some cases where temperaturerequirements are not too severe, metals such as iron, nickel, chromia,etc. ln some processes, pebbles which will withstand temperaturesupwards of 3000? F. are required and several good pebbles which functionat these temperatures have been developed, one of the most suitablebeing mullite-alumina. Pebbles range in size from about 1/s to l" indiameter, but usually are within the range of 1A to s/s.

One of the problems involved in pebble furnace operation and design isin avoiding the overheating and too rapid heating of pebbles withresulting thermal shock and fracturing of the pebbles which renders themunfit for pebble heater use. The avoidance of subjecting pebbles tosevere thermalv shock poses a difficult problem in pebble heateroperation. The problem of bringing the pebble bed up to operatingtemperature after a 2,776,825 Patented Jan. 8, 1957 ICC shutdown or atany other time when the pebbles in the lower section of the furnace havedropped in temperature considerably below operating temperature withoutsevere thermal shock is particularly difficult. The hot combustion gasat temperatures of'upwards of 2500 F. when contacting relatively coldpebbles, subjects them to extremely severe thermal shockv and causes thefracturing of a portion of the pebbles.

Another problem of pebble furnace design is in designing the furnace soas to obtain uniform pebble flow therethrough. Uniform pebble flow inthe pebble heating chamber is important because without it, over-heatingof some of the pebbles will occur while some sections of the gravitatingpebble mass will be underheated. Conventional design provides acylindrical chamber with a hopper or funnel-shaped bottom which directsthe flow of pebbles into a relatively narrow throat leading to the lowerheat-exchanger. This corresponds to the general shape of the lowerheat-exchanger also. It is found that pebble flow in this type of vesselis non-uniform through a considerable portion of the Vessel. Stagnantflow areas exist chiefly around the juncture of the conical bottom andthe cylindrical sides of the vessel, while extremely rapid flow areasare found around the axis of the vessel. This difficulty of obtainingnon-uniform flow of contact material in a cylindrical vessel isemphasized in the U. S. Patent 2,430,669, to John A. Crowley. While thepatent is not concerned with pebble heatexchanger operation but ratherwith the flow of irregular particulate contact material, the flowproblems are similar.

Another difficulty encountered in pebble heater operation and designlies in obtaining uniform gas flow upwardly through all sections of theheat-exchanger chamber. Without uniform gas flow, it is obvious thatthere will be a lack of uniform heating, and especially treating orreacting of the gas in the lower chamber. In the conversion ofhydrocarbons, particularly, uniform gas ow is paramount in order tocontrol the exact contact time required in most conversion processes andespecially processes which are effected at the high temperaturesprovided in pebble heater processes. It has been found thatover-contacting or soaking of the hydrocarbon in the heat-exchangerresults in complete cracking to coke and deposition of the same on boththe pebbles and the interior of the chamber which has sometimes resultedin shutting.y down of the unit due to clogging of the pebble passagewayswith fragments of coke from the walls of the exchanger or agglomerationof the pebbles with carbon.

The principal object of the present invention is to provide a methodand. apparatus which substantially reduces thermal shock involved inheating the pebbles in a pebble furnace up to operating temperatureafter shutdowns and other interruptions and in effecting other requiredchanges iny pebble temperatures. It is also an object of the inventionto provide uniform pebble and gas flow in a pebble heater system.Another object of the invention is to provide uniform contact betweengas and pebbles in a pebble heat-exchanger. A further object of theinvention is to provide uniform gas-pebble contact with a minimum ofpressure drop through `the pebble bed. A stillV further object is toprovide a means and method of introducing secondary air into thecombustion gas being fed to the pebble bed in a pebble furnace. Otherobjects of the inventionl will become apparent from a consideration ofthe accompanying disclosure.

The instant invention provides a novel means and method forregulatingthe temperature of the heating gas in a pebble furnace whereinsecondary air is admixed with the combustion gas' intermediate theburner (or burners) and the ports. or gas distributing means which feedthe heating gas to the lower section of the pebble bed.

The invention also provides a pebble furnace design which requiresrelatively short horizontal gas ow through the bed and therefore permitsthe gas to traverse the whole horizontal cross-section of the bed on itsway upwardly through the heating chamber. It has been found that it isadvantageous to introduce the hot pebbles from a pebble furnace into thelower heat-exchange chamber through a plurality of pebble inlets aroundthe periphery of the heat-exchange chamber. This method of introducingpebbles to the lower chamber provides a more uniform pebble bed surfaceand therefore more uniform flow of pebbles and of gas to be heated orreacted in the lower bed with resultant uniformity of heating and/ orconversion of the gas being treated.

The furnace of the invention in one modification provides an axialcombustion chamber below the floor of the heating chamber and a seriesof tangential, generally lateral or horizontal passageways leading intothe combustion chamber for introducing and mixing secondary air with thecombustion gas in any desired proportion so as to regulate thetemperature of the mixture in accordance with the temperature of thepebbles in the bottom of the heating chamber and with the desired changein the pebble temperature. These tangential passageways or conduitscommunicate with an annular space between the metal shell of the furnaceand the refractory surrounding the combustion chamber which has asuitable air supply means so as to provide the required secondary air.This arrangement or design reduces heat losses from the combustionchamber and aids in preventing overheating of the refractories. Anotheradvantage of the structure is found in the utilization of a singleburner and combustion chamber to supply a plurality of ports and gasdistributing members with the hot combustion gas mixture required forheating the pebbles. A second modification utilizes a separate burnerfor each gas distributing member with one or more secondary air inletconduits leading into each combustion chamber.

In order to improve the ow of pebbles through the heat-exchanger, thepresent design provides a relatively narrow pebble column above eachpebble outlet. It has been found that in order to obtain any degree ofuniformity of pebble flow, even in the upper section of the bed, theheight of the bed must be at least one and onehalf times the horizontalwidth of the bed in a vessel where the pebble outlet is positioned atthe center of the bottom. By effectively dividing up the gravitatingpebble mass into several narrow columns with a separate pebble outletfor each, the effective height of the column is increased and its widthdecreased, and it is therefore found possible to utilize each column asa uniformly gravitating mass for contact with the contacting gasadmitted at points near the bottom of the column. The uniformlygravitating section of ythe pebble bed in the apparatus of the inventionincludes approximately the upper seven-eighths of the bed. The highnarrow columns of flowing pebbles provide maximum uniformity of flowwithout `cutting down the effective width of the entire pebble bed and,hence, the capacity of the unit. The etfective narrowing of the pebblecolumn is combined with the injection of contacting gas at points closerto the center of the column than is provided in conventional pebbleheat-exchangers.

In order to provide a more complete understanding of the invention,reference is made to the drawing of which Figure 1 is an elevation,partly in section, on line 1 1 of Figure 2, showing, somewhatdiagrammatically, a pebble furnace with a subjacent gas heating and/orreaction chamber; Figure 2 is a horizontal cross-section of the furnaceof Figure l taken on the line 2-2; Figure 3 is a horizontalcross-section through the combustion chamber of the furnace of Figure ltaken on the line 4 3-3 thereof; and Figure 4 is a cross-section of upartial section of a combustion chamber and its gas distributing meansin accordance with one modification of thc invention.

Figure l shows a pebble furnace 1t) in axial alignment with a subjacentgas heating or reaction chamber 1l. The furnace and heater may have acommon shell 12 which is lined with suitable refractory materials 13 and14 in two or more layers. The inner refractories in the lower section ofthe furnace and around the combustion chamber as well as in the uppersection of the gas heating chamber are constructed of super refractorieswhich will withstand extremely high temperatures.

Pebbles are fed into the furnace from an elevator 16 through chute 17and pebble inlet conduit 18. The upper end of pebble inlet conduit 18functions as a combustion gas stack or flue. An expansion of this pebbleconduit where it joins the top of the furnace cooperates with a pebbledistributor 19 in passing the pebbles to the distributor and allowing aportion of the combustion gas from the furnace to bypass the pebbledistributor through an annular space 21 and to pass thru the incomingpebbles and out conduit 18. A stack 22 in the top of the furnace carriesotf the remainder of the combustion gas. With a stack damper in stack 22(not shown) this design permits removal of fines from the system byentrainment in the flue gas withdrawn thru 18.

The function of the pebble distributor is to deliver the incomingpebbles rather uniformly over the top of the pebble bed so as to providemore uniform distribution and more uniform flow of pebbles. Thedistributor may have any suitable number of arms or delivery spouts. The`modlcation in Figure l, shows a hollowl center column 23 having ports24 in its lower end and a burner 26, supplied by one or more conduits30, at its base. This modi'cation is desirable in furnaces of ratherlarge diameter so as to improve the uniformity of pebble flow and gasflow therein. It is particularly advantageous to use a center columnwith combustion gas distributing means therein as well as a combustiongas source in furnaces having four or more gas distributing sections orelements around the periphery of the furnace wall and an equal number ofpebble outlet conduits. However, it is not essential to have a centercolumn in relatively small furnaces and the center column can be solidor hollow and imperforate where distribution of combustion gas from theaxis of the heating chamber is not essential. A floor 27 separates thefurnace into a heating chamber 28 and a combustion chamber 29. Aplurality of pebble conduits 31 extend downwardly thru the floor of thefurnace and feed hot pebbles into gas heating chamber 32. Each pebbleoutlet conduit cooperates at its upper end with a funnel or cone shapeddepression 33 in the combustion chamber oor so as to expedite the How ofpebbles out of the pebble column above the funnel or cone and itsadjacent area.

A series of gas distributing sections 34 are spaced uniformly around theoor of the heating chamber and serve to introduce the combustion gasmixture to the bottom of the pebble bed. These gas distributing sec--tions are made up of a vertical wall of refractory brick checker work inthe form of an arc of a circle projecting inwardly from the refractorywall of the heating chamber. A downwardly and inwardly slopingrefractory roof or cap mortised into the wall of the heating chamberexpeditcs the flow of pebbles over the gas distributing section andprevents the passage of pebbles into the port 37 which conducts gas tothe distributing section. These ports 37 communicate through a series ofconduits 38 with combustion chamber 29 and provide means for passing thecombustion gas-air mixture from the combustion-mixing chamber 29 to thegas distributing section 34.

The number of gas distributing sections required in a pebble furnacedepends upon the diameter of the furhace and on, Whether or not a centecombustion column is used and the size thereof as well as upon theuniformity of gas distribution desired in the pebble heating chamber. Atleast three gas distributing sections are desirable in most furnaces atleast four gas distributing sec tions, are advantageous.

The heater of Figure l utilizes a series of secondary air inlets 39,which are preferably tangentially arranged as shown in Figure 3, and maybe advantageously inclined upwardly toward the combustion chamber outletend so as to aid inimparting spiral gas flow and mixing of the secondaryair with the combustion gas as it passes upwardly through the combustionchamber to outlet ports or conduits 38. Air inlet conduits 39communicate with annular manifold space 41 surrounding the combustionchamber refractory inside shell 12. One or a plurality of air inletconduits 42 serve to supply air to the manifold.

The refractory wall of the furnace above manifold 41 is supported by anannular' steel plate 43 welded to the shell and supported by steel anglebraces 44 spaced around the bottom of plate 43 at suitable intervals.Other suitable means may be used for supporting the refractory wall ofthe furnace.

In the modification of the invention shown in Figure l, a single burner46 in the bottom of combustion chamber 29 is utilized to supply the hotcombustion gas for the furnace. Lines 47 and 48 supply air and fuel,respectively, to burner 46.

Pebble outlet conduits 31 extend directly into gas heating chamber 32which is formed by the refractory wall of the unit extending down fromthe furnace, by the roof or dome 49, and by conical bottom 51. A gastakeoff 50 in dome 49 serves to remove product gas. The height of thegas heating or reaction chamber may be 'adjusted to suit the type ofprocess to be effected in the apparatus. The gaseous mixture fed to theheating charnber or reactor is passed throughv line 52 to manifold space53 formed between the conical bottom of the chamber and the bottom ofshell 12. The feed gas passes from manifold space 53 through a series ofgas distributing conduits 54 and inverted troughs 56 which form, a gasdistributor in the bottom of chamber 32. Pebble outlet conduit 5'/passes pebbles from the reaction chamber to a downwardly sloping chuteleading into an elevator 16 which delivers pebbles to chtite 17 at thetop of the :apparatus. The elevator may be a bucket type, a gas lift, orany other suitable means for transferring pebbles from the bottom to thetop of the apparatus.

Figure 2 shows the floor plan of the pebble heating chamber of thefurnace. The modification shown utilizes four gas distributing sections34 and a corresponding number of pebble outlet conduits 31 passingthrough the floor of the chamber. rThe arcuate wall of the gasdistributing section is preferably made up of a continuous refractory 5bsupported by tapered refractory bricks 58. By utilizing a. continuousrefractory slab such as 59, refractory bricks 58 can be spaced ratherfar apart so as to provide maximum open space for gas distribution. Byutilizing this type of construction the vertical face of the gasdistributor may contain as much :as 70 percent open space. Of` course, aplurality of continuous refractories 59 with the requiredz separatingand supporting bricks may be utilized in each distributor to obtain thenecessary open space for gas distribution. Figure 2 bear the samenumeral designations as those shown in Figure l and further description`of the figure is believed unnecessary.

Figure 3 is a cross section of the furnace taken on line 3 3 of Figure 1and shows a. plan of the secondary air supply and distribution system inrelation yto thev combustion and mixing chamber 29. This section of theapparatusshows to `advantage the tangential arrangement of Vsecondaryair inlet conduits 39 in relation to combustion chamberv 29, manifoldspace 41, and pebble outlet The other elements of e conduits 31. Thetangential arrangement of the secondary air inlets, while improving themixing of the air and combustion gas, is not` essential to theinvention. Introduction of the air radially is also feasible and withinthe scope of the invention. It is also feasible to introduce thesecondary `air directly from a source outside of the unit.

Figure 4 shows a modification of the gas distributing structure which`may be utilized in the furnace yof Figure l, or with the combustionchamber structure shown in Figure 4. Likewise, the gas distributingstructure of Figure l may be utilized with the burner and secondary airarrangement of Figure 4. The structure of Figure 4 utilizes a gasdistributing section 61 which is a vertical section of a cone cut by facylinder, the cylinder being the inner wall of the pebble heatingchamber. Section 61 may also be an oblique section of a cylinder cut bythe cylindrical chamber. A sufficient number of radially disposedinwardly and downward sloping holes or ports 62 in element 6l serve todistribute gas to the lower section of the pebble bed in the pebbleheating chamber. lt is advantageous in obtaining uniform gasdistribution to position the openings 62 in elem-ent 61 and the openspaces in the gas distributing section 34 of Figure 1 radially withrespect to the port under the gas distributor. In this way gas isdirected outwardly over the whole area of the pebble bed surrounding thegas distributor so as to result in more uniform heating than wouldotherwise be provided. In the modification shown in Figure 4 a burner 63positioned in a housing 60 directs combustion gas into a combustionchamber 64'which opens into the oor 27 of the furnace thereby providinga passageway for combustion gas from the combustion chamber to thedistributing section 61. One or more secondary air supply lines orconduits 66 open into. combustion chamber 64 and supply air thereto formixing with and tempering the combustion gas as desired.

The apparatus o-f the invention has numerous advantages overconventional pebble furnaces and pebble heater arrangements. T hestructure of Figure 1 is compact and relatively simple so as to renderthe apparatus comparatively easy to manufacture and assemble. Thecombustion chamber and the secondary air supply system are incorporatedinside the shell so as to simplify the shell structure. The pebblefurnace can be operated at practically any temperature since the burneruses av pre-mix and the secondary air is used as a diluent to reducetemperature. This is particularly advantageous at start-up from coldcondition in order to prevent thermal shock to both the refractories andthe pebbles. Temperature of the heating gas at start-up can be held toSOO-350 orv 400 F. until the refractories are warmed up approximately tothe gas temperature at which time the gas temperature can be graduallyincreased over a consider-l able period of time by decreasing theproportion of secondary air in the gas mixture either by decreasing .theamount of secondary air introduced into the combustion chamber or byincreasing the amount of combustion gas. The secondary `air distributorand manifold are also integral parts of the furnace incorporated withinthe shell. Tubes imbedded within the castA refractory carry thesecondary air to the combustion chamber. The bottom of the heatingchamber slopes toward the pebble outlets to prevent stagnant area-s ofthe pebble bed with resulting non-uniformity of heating. The extensionof the gas distributing section only a short distance up the side of thefurnace and the sloping roof thereon make it possible to utilize thefull diameter of the chamber for heating the pebbles while stillobtaining relatively uniform pebble flow through the bed. The elongatedcharacter of pebble conduits 31 is advantageous in preventing gas flowfrom either pebble heating chamber 28 to gas'heating chamber 32 or viceversa. In this manner these pebble conduits function as valves toprevent gas flow in either direction when filled with pebbles, as theyof necessity are, during pebble heater operation.

The effective narrowing of the pebble bed provided by the invention isobvious from a consideration of the designs shown in the various figuresof the drawing, and this is effective in reducing the horizontaldistance which the combustion gas must travel in order to traverse theentire cross-section of the pebble bed. This horizontal distance isshortened by application of the invention to a pebble heat-exchanger ofany given diameter.

Pebble heat-exchangers of the type with which this invention isconcerned have utility in a wide variety of heat-exchange processeswhich require the heating, treating, and/or reaction of a gaseous feed.The application of M. O. Kilpatrick, Serial No. 761, 696, tiled July 17,1947, now abandoned, describes the detailed operation of a pebbleheat-exchanger unit in the conversion of hydrocarbons. The applicationof C. W. Perry, Serial No. 677,357, filed June 17, 1946, now Patent No.2,596,507, shows the use of pebble heat-exchangers in the synthesis ofHCN from NH3 and carbon-containing gases. The application of Sam P.Robinson, Serial No. 665,673, filed April 29, 1946, now Patent No.2,551,905, illustrates the use of pebble heat-exchanger apparatus in thedesulfurization of gases, particularly hydrocarbon gases. The use ofpebble heat-exchangers in the synthesis of carbon disulfide fromhydrocarbon and sulfur-containing gases is disclosed in the applicationof Sam P. Robinson, Serial No. 651,293, filed March l, 1946, nowabandoned. The heating of air for high temperature use in the fixationof atmospheric nitrogen in a pebble heat-exchanger is disclosed in theapplication of Sam P. Robinson, Serial No. 767,300, filed August 7,1947, now abandoned. The process of this last application illustratesthe use of a heating gas other than combustion gas for heating thepebbles in the upper heat-exchanger. In the process referred to, airheated in the lower heat-exchanger is mixed with fuel and burned in ahigh temperature furnace to produce a temperature of approximately 4000F. so as to effect the fixation of nitrogen, and, in order to preventdecomposition of the product, the hot effluent is irnmediately passedthrough the upper heat-exchanger in contact with relatively coldpebbles, thereby heating the pebbles and quenching the product effluentfrom the reaction in the furnace. The hot pebbles gravitate to the lowerheat-exchanger and are there contacted with atmospheric air so as topre-heat the air for the fixation process. The apparatus of theinvention i-s readily adaptable to the nitrogen fixation process byintroducing the gas to be quenched directly to the ports 37 of theapparutus.

The instant invention is applicable to the processes which theapplications just referred to relate. Since the pebble heat-exchangerdisclosed provides uniform pebble flow and gas flow, it is particularlyapplicable to the con version of hydrocarbons and to chemical processeswhich require short, specific reaction times. ln hydrocarbon conversionprocesses involving dehydrogenation and cracking to specific productswithout over-cracking, the present invention offers particular utility.

As will be apparent to those skilled in the art, various modificationsof the invention, in addition to those disclosed, are within the scopeand spirit of the disclosure and claims.

I claim:

l. A pebble furnace for heating a gravitating compact bed of pebbles,comprising a refractory-lined upright closed cylindrical vessel havinggas outlet and pebble inlet means in its top; a lioor in said furnaceintermediate the top and bottom thereof forming a pebble heating chamberbetween said floor and said pebble inlet means, said fioor having atleast three upwardly extending ports therein adjacent the vertical wallof said vessel and being imperforate except for said ports and thehereinafter described pebble outlet conduits; means for supplying hotcombustion gas to each of said ports including burner means, combustionchamber means communicating with said burner means, and conduit meansconnecting said combustion chamber means witheach of said ports; aseries of separate inwardly convex gas distributing sections enclosingthe outlet ends of said ports and extending only a portion of the heightof said chamber, said sections being perforate and adapted to passcombustion gas into said chamber from said ports; conduit meansintermediate said burner and said ports for introducing secondary airinto the combustion gas passing thru said ports; and at least threepebble outlet conduits extending thru said fioor, spaced alternatelywith respect to said gas distributing sections and intermediate the walland the center of said chamber.

2. The furnace of claim l in which said gas distributing sections are inthe form of a vertical segment of a cone cut by a cylinder with the baseat the port and the apex on the wall of said vessel.

3. The furnace of claim 1 in which said gas distributing sections are inthe form of an upright wall of refractory checkerwork having an inwardlyand downwardly sloping imperforate roof mortised into the wall of saidvessel.

4. A pebble furnace for heating a gravitating cornpact bed of pebbles,comprising a refractory-lined upright closed cylindrical vessel havinggas outlet and pebble inlet means in its top; a oor in said furnaceintermediate the top and bottom thereof forming a pebble heating chamberbetween said oor and said pebble inlet means, said floor having at leastthree upwardly extending ports therein adjacent the vertical wall ofsaid vessel; an axial refractory-walled combustion chamber below saidfloor having a burner in its lower section; tubes for combustion gasleading from said combustion chamber to each of said ports; conduitmeans leading into said combustion chamber intermediate said burner andsaid tubes for introducing secondary air to said combustion chamber; aseparate inwardly convex gas distributing section enclosing each of saidports and extending only a portion of the height of said chamber, saidsection being perforate so as to pass combustion gas into said chamberwhile avoiding passage of pebbles to said port; a pebble outlet conduitin said fioor intermediate each adjacent pair of gas distributingsections and intermediate the wall of said vessel and the axis thereofextending downwardly through the wall of said axial refractory-walledcombustion chamber to an expanded pebble chamber; and a funnel-shapeddepression in said floor around each said pebble outlet for facilitatingpebble fiow out of said chamber.

5. The furnace of claim 4 in which said conduit means includes a seriesof tangentially and symmetrically dis posed passageways leading intosaid combustion chamber thru the refractory wall thereof from an airsource.

6. The furnace of claim 5 including an annular open space between saidrefractory wall and the outside wall of said vessel and at least oneopening thru said outside wall into said open space for introducing airand providing said air source.

7. The furnace of claim l including a hollow upright cylindricalrefractory column disposed axially on said floor and extending into theupper section of said chamber; a burner in said hollow column forproviding cornbustion gas; and downwardly radially directed ports in thewall of said column in the lower section thereof for passing combustiongas from said column to said chamber.

8. The furnace of claim l including a combustion chamber leading intoeach of said ports thru the wall of said vessel; a burner in the outerend of said combustion chamber; and a secondary air inlet in saidcombustion chamber intermediate said burner and said port.

9. The method of tiring a pebble furnace containing a compact bed ofpebbles which comprises burning a fuel so as to produce combustion gas;mixing with said combustion gas a secondary air stream so as to decreasethe temperature of the mixture, thereafter passing said mixture upwardlythru said bed of pebbles in direct heatexchange therewith so as to heatsame with less thermal shock than would occur without the mixing step.

10. The method of tiring a pebble furnace containing a compactcylindrical bed of pebbles which comprises burning a combustible mixtureof oxygen and fuel in an axial cylindrical combustion zone below saidbed; introducing secondary air tangentially to said combustion zone soas to form an intimate mixture of air and combustion gas of lowertemperature than said combustion gas and of higher temperature than saidpebbles; passing said mixture to a series of uniformly spaceddistributing Zones around the outer periphery of the lower section ofthe said bed; distributing said mixture radially outwardly and laterallyfrom each of said distributing zones into the lower section of said bed;thereafter, passing `said mixture upwardly thru said bed in -directheat-exchange with the pebbles therein so as to heat same to the desiredtemperature.

1l. The combination of the pebble furnace of claim 4 with a subjacentaxially disposed refractory lined heatexchange chamber in which thepebble outlet conduits open into the upper end of said heat-exchangechamber and serve as pebble feeder conduits thereto, said heatexchangechamber having pebble outlet means and gas inlet and distribution meansin its lower end and gas takeoff means in its upper end.

12. The furnace of claim l in which said gas distributing sections arein the form of an oblique section of a cylinder fitting the inside wallof the furnace.

13. The furnace of claim 4 wherein said burner is upwardly directed andpositioned at the axis of said combustion chamber.

14. A method of bringing a cool bed of pebbles in a pebble heatingchamber up to operating temperature with a minimum of thermal shock tothe pebbles which comprises burning a combustible mixture of oxygen andfuel in an axial combustion zone below said bed; introducing secondaryair tangentially to said combustion Zone so as to form an intimatemixture of air and combustion gas initially at a temperature in therange of 300 to 400 F.; passing said mixture to a series of uniformlyspaced distributing zones around the outer periphery of the lowersection of said bed; distributing said mixture radially outwardly andlaterally from each of said distributing zones into the lower section ofsaid bed; thereafter, passing said mixture upwardly through said bed indirect heat-exchange with the pebbles therein so as to` heat same to thedesired temperature; and gradually increasing the temperature of saidmixture by increasing the proportion of combustion gas therein as 'thetemperature of said pebble bed is increased, until the desired operatingtemperature is reached.

l5. A method of bringing a cool bed of pebbles in a pebble heatingchamber up to operating temperature with a minimum of thermal shock rtothe pebbles which comprises burning a combustible mixture of oxygen andfuel in a combustion zone outside of said bed; introducing a secondaryair to said combustion Zone so as to form an intimate mixture of air andcombustion gas initially at a temperature in the range of 300 to 400 F.;passing said mixture to a series of uniformly spaced distributing zonesaround the outer periphery of the power section of said bed;distributing said mixture radially outwardly and laterally from each ofsaiddistributing zones into the lower section of said bed; thereafter,passing said mixture upwardly through said bed in direct heat-exchangewith the pebbles therein so as to heat same to the desired temperature;and gradually increasing the temperature of said mixture by increasingthe proportion of combustion gas therein as the temperature of saidpebble bed is increased, until the desired operating temperature isreached.

16. A method of bringing a cool bed of pebbles in a pebble heatingchamber up to operating temperature with a minimum of thermal shock tothe pebbles which comprises burning a combustible mixture of oxygen andfuel in a plurality of combustion zones positioned around the peripheryof a lower section of a bed of pebbles; introducing secondary air intoeach of said combustion zones so -as to form an intimate mixture of airand combustion gas initially at a temperature in the range of 300 to 400F.; passing said mixture to a series of uniformly spaced distributingZones around the outer periphery of the lower section of said bed;distributing said mixture radially outwardly and laterally from each ofsaid distributing Zones into the lower section of said bed; thereafter,passing said mixture upwardly through said bed in direct heat-exchangewith the pebbles therein so as to heat same to the desired temperature;and gradually increasing the temperature of said mixture by increasingthe proportion of combustion gas therein as the temperature of saidpebble bed is increased, until the desired operating temperature isreached.

References Cited in the file of this patent UNITED STATESl PATENTS2,272,108 Bradley Feb. 3, 1942 2,285,718 Isaacson June 9, 1942 2,505,257Quigg Apr. 25, 1950 2,534,625 Robinson Dec. 19, 1950 2,536,436 GoinsJan. 2, 1951 2,563,323 Grossman Aug. 7, ll 2,635,950 Robinson Apr. 21,1953 2,635,990 Goins Apr. 21, 1953

